The production of liquefied natural gas (LNG) may be achieved by cooling and condensing a feed gas stream against multiple refrigerant streams provided by recirculating refrigerant systems. Cooling of the natural gas feed may be accomplished by various cooling process cycles, such as the well-known cascade cycle in which refrigeration is provided by three different refrigerant loops. One such cascade cycle uses methane, ethylene, and propane cycles in sequence to produce refrigeration at three different temperature levels. Another well-known refrigeration cycle uses a propane pre-cooled, mixed refrigerant (C3MR) cycle in which a multicomponent refrigerant mixture generates refrigeration over a selected temperature range. The mixed refrigerant may contain hydrocarbons such as methane, ethane, propane, and other light hydrocarbons, and also may contain nitrogen. Versions of this refrigeration system are used in many operating LNG plants around the world.
These and other types of refrigeration processes for natural gas liquefaction involve the use of refrigerant compressors driven by gas turbines. In recent years, single-shaft gas turbines have been used for this purpose. During a blocked compressor discharge event, the compressed refrigerant typically is discharged to a flare system, which must be sized to handle anticipated relief flows during such an event. There also are other systems in the prior art, some of which are discussed below.
The term “baseload LNG plant” as used herein is intended to mean a facility that continuously produces liquefied natural gas via refrigeration from at least one of the many cooling process cycles known in the art. The facility may be a land-based site, a floating production, storage, and offloading (FPSO) facility to recover natural gas from the sea/ocean floor, or a gravity-based system (GBS), a FPSO site that is anchored to the sea floor in a particular location.
Persons skilled in the art will understand that the net power available from a gas turbine used to drive a refrigerant compressor in a baseload LNG plant is a function of several variables, including but not limited to ambient temperature (the most power is available at cold temperatures), inlet/outlet duct losses, frictional losses, compressor fouling over time, etc.
U.S. Pat. No. 4,799,359 (Nicoll) discloses a cryogenic refrigeration compressor containing an externally adjustable relief valve between the compressor discharge and the suction lines. This allows cryogenic fluid to flow from the compressor discharge to the suction line when the discharge pressure exceeds a set value.
U.S. Pat. No. 4,566,885 (Haak) discloses a liquefaction process with two closed loop refrigeration cycles. In each loop, the compressors are driven by a gas turbine. At times of low power consumption by the compressors at the first loop, turbine power is diverted to a generator. The generator supplements the power generated by the turbine of the second loop.
International Publication WO 88/06674 discloses a process, applicable to FPSO and stationary platforms, to relieve high pressure gas discharge to the sea floor. Any low and/or medium pressure gas is discharged through a conventional flare. This reduces the necessary diameter and length of the flare extending from the platform.
International Publication WO 97/33131 discloses a liquefied natural gas process characterized by coolant loop compressors being mechanically interconnected and driven by a single-shaft gas turbine. Also disclosed is a bypass valve between the inlet and outlet of each compressor for use during process start-up.
U.S. Pat. No. 5,408,840 (Talley) discloses a refrigerant recovery process. In the event of refrigeration circuit overpressure, the refrigerant, after passing through the pressure relief valve, is collected in a vessel rather than being vented to the atmosphere.
U.S. Pat. No. 5,319,945 (Bartlett) discloses a process where, in the event of overpressure, refrigerant is diverted from the refrigeration loop to a holding vessel. The volume of the holding vessel must be large enough to reduce the overpressure before a relief vent set pressure is reached.
U.S. Pat. No. 3,855,810 (Simon et al.) discloses the insertion of a sufficient buffer volume on the low-pressure side of a refrigeration circuit to accommodate the build-up in pressure when the compressor is cut-off. This eliminates the need to flare refrigerant through pressure relief valves.
As described above, most refrigerant compressors used in baseload LNG plants are driven by gas turbines. In many applications, single shaft gas turbines are used wherein the gas turbine and one or more compressors or compressor stages are mounted on a single shaft. If the compressor discharge is blocked by an unexpected process event, the compressed refrigerant typically is discharged to a piping and flare system, which must be sized to handle anticipated relief flows during such an event. It is desirable to minimize the size of the piping and flare system required to handle such flows. It also may be desirable for economic and environmental reasons to minimize the amount of gas flared during process upsets or compressor discharge blockage events.
Embodiments of the present invention address these needs and include an apparatus and method for minimizing relief flows in baseload plants for the production of liquefied natural gas (LNG), relating in particular to an apparatus and method which take advantage of the bogdown characteristics of single-shaft gas turbines used to drive refrigerant compressors in order to minimize flare loading during a blocked compressor discharge event.